1. Field of the Invention
The present invention relates to the development of underground media such as petroleum reservoirs. More particularly, the invention relates to a history matching method for a geological model representative of an underground reservoir, wherein the geometry of a fault network is adjusted in order to reproduce by simulation the observed data.
2. Description of the Prior Art
Studying a petroleum field requires constructing models referred to as “geological models” in a broad sense. These models, which are well known and widely used in the petroleum industry, allow determination of many technical parameters relative to, for example, prospecting, study or development of a hydrocarbon reservoir. In fact, the geological model is representative of the structure of the reservoir and of the behavior thereof. It is thus for example possible to determine which zones are the likeliest to contain hydrocarbons, the zones in which it can be interesting/necessary to drill an injection well in order to enhance hydrocarbon recovery, the type of tools to use, the properties of the fluids used and recovered, etc. These interpretations of geological models in terms of “technical development parameters” are well known, even though new methods are regularly developed. It is thus crucial, in the petroleum field, to construct a model as precisely as possible. Integration of all the available data is therefore essential.
A geological model is a model of the subsoil, representative of both the structure and the behavior thereof. Generally, this type of model is represented in a computer. It is then referred to as a numerical model. In two dimensions (2D), it is described as a map. Thus, a map corresponds to an image of pixels with each pixel containing information relative to the behavior of the subsoil being studied (a petroleum reservoir for example). These pixels correspond to a precise geographical position and they are identified by coordinates. When values are assigned to a pixel, by simulation for example, they are referred to as simulation points. The representative image (map or model) is generated on any support (paper, computer screen, etc.).
Petroleum reservoirs are generally highly heterogeneous and fractured porous media. In order to obtain the best possible image of the reservoir, it is therefore necessary to integrate the faults, in addition to the static and dynamic data.
A fault is a surface generated by a shear fracture separating the rock by creating a throw between two adjacent blocks. There are two types of faults in a reservoir. Seismic faults are large objects visible from seismic velocity surveys. They are objects of large size (several hundred meters to some kilometers). Sub-seismic faults are objects whose size is not sufficiently large for them to be visible in seismic images.
A geological model is a representation of the petroleum reservoir where the geometry of the faults is generally represented by Boolean objects. In two dimensions, the faults are represented by lineaments, in three dimensions by surfaces. Properties such as porosity, permeability, effective opening, etc., are associated with each object.
The seismic faults are added in a deterministic manner, as seen in seismic surveys. The sub-seismic faults that are invisible are added using construction methods referred to as “probabilistic” due to the limitation of available information (limited number of wells, etc.). The geological models constructed from these probabilistic methods are therefore referred to as “stochastic models”.
Fractal methods can be mentioned among others to take into account the auto-similar character for constraining the stochastic model.
Since it is impossible to directly simulate the flow in an object model (that is a geological model where the faults are represented by objects), the faults are discretized on a simulation grid. For each cell of the flow model, the properties equivalent to the presence of a fault (permeability, porosity, etc.) are calculated. This simulation grid allows obtaining simulated production data, that is data which are compared with the real data. If the difference is too great, the geological model is modified iteratively until the simulations are comparable with the observations.
The object of the method is to gradually deform the fault network with a view to matching.
There is a method allowing hydrodynamic matching of a fracture network in relation to production data. It is based on the gradual deformation of a Boolean model by acting upon the Poisson drawing of the position of the objects. This method is described in U.S. Pat. No. 7,483,822 or French Patent 2,857,764. The application to fracture models is described in:    Lin Y. Hu and Sandra Jenni: History Matching of Object-Based Stochastic Reservoir Models. SPE Journal, 10 (3) no 81503, September 2005.
In this method, the positions of all the faults are modified to reproduce the production history. The displacement of the faults is controlled by a reduced number of parameters, which can then be optimized to reduce the difference between simulated data and production data.
This method however involves several drawbacks. First, the input parameters of the algorithm that generates the faults are arbitrary and cannot be related to field observations. Second, the models generated are not realistic from a geological point of view.
Fractal methods are frequently used to model faults by stochastic methods. The latter allow generation of networks of auto-similar objects on various observation scales. The advantage of these methods is to generate realistic objects. Such methods are described in:    M C. Cacas, J M. Daniel, and J. Letouzey. Nested Geological Modelling of Naturally Fractured Reservoirs. Petroleum Geoscience, 7:S43-S52, 2001,    Bour, O and Ph. Davy, Connectivity of Random Fault Networks Following a Power Law Fault Length Distribution, Water Resour. Res. 33 (7), 1567-1583, 1997.
The drawback of these methods is that they do not allow modification of the fault generated networks to perform hydrodynamic matching.